In digital transmission of information, it is desired to assess signal quality for a variety of reasons. For example, channel assignment decisions in trunked communication systems, such as cellular telephone systems or trunked dispatch systems, are carried out by some form of central controller in response to channel requests from a plurality of communication units. With a signal quality measurement for each of the available channels in the system, the central controller can avoid undesirable channel conditions such as co-channel interference from other trunked or conventional systems. Such a measurement is also useful in selecting particular transmission lines for wireline or fiber optic transmissions.
The communication units, also called subscribers, are arranged in talk groups and may be located anywhere within the system (in the coverage area of any base site). At times, when a mobile communication unit is active in a group or individual call, the mobile unit travels out of the coverage area of its current site, causing the call to become degraded and even lost. The call may then be handed off to another site for completion. Use of signal quality measurements help make call hand-offs less troublesome by avoiding channels with high levels of noise. Thus, when determining if a channel is available at a site, it is desirable for the central controller to assign a channel with the least amount of co-channel interference, such that the present call may be continued without interruption or degraded reception. A number of methods of channel selection exist, including signal detection and signal quality measurement techniques.
An existing method of signal detection starts a timer as soon as a simple carrier is detected (above a preset signal level threshold) on one of the inbound voice channels. The timer continues to run as long as the carrier remains present. A break in the carrier resets and turns off the timer. When the timer expires, i.e., reaches a predetermined value, say, 50 seconds, the channel is considered to have undesired interference and is taken out of service for a period of time, such as 5 minutes. This carrier detection method is best suited to detect the class of interference called intermodulation, which is very common at more congested antenna sites. It is not suited to instantaneously and reliably detect the presence of a true co-channel user.
Signal quality measurement techniques include bit error rate (BER) measurement, in which the number of digital data bits that are incorrectly received are counted, and signal-to-noise ratio (SNR) measurement, which takes the ratio of signal strength to noise strength. BER and SNR measurements tend to be non-linear and do not give early detection, e.g., at low BER or high SNR, of co-channel interference because the measurement techniques are slower than what is needed for an expeditious hand-off or channel assignment.
Accordingly, there is a need for a method of co-channel interference measurement that can quickly and reliably measure co-channel interference.